Calcineurin and Its Role in Synaptic Transmission.

Calcineurin (CaN) is a serine/threonine phosphatase widely expressed in different cell types and structures including neurons and synapses. The most studied role of CaN is its involvement in the functioning of postsynaptic structures of central synapses. The role of CaN in the presynaptic structures of central and peripheral synapses is less understood, although it has generated a considerable interest and is a subject of a growing number of studies. The regulatory role of CaN in synaptic vesicle endocytosis in the synapse terminals is actively studied. In recent years, new targets of CaN have been identified and its role in the regulation of enzymes and neurotransmitter secretion in peripheral neuromuscular junctions has been revealed. CaN is the only phosphatase that requires calcium and calmodulin for activation. In this review, we present details of CaN molecular structure and give a detailed description of possible mechanisms of CaN activation involving calcium, enzymes, and endogenous and exogenous inhibitors. Known and newly discovered CaN targets at pre- and postsynaptic levels are described. CaN activity in synaptic structures is discussed in terms of functional involvement of this phosphatase in synaptic transmission and neurotransmitter release.

CaMKII Is Involved in the Choline-Induced Downregulation of Acetylcholine Release in Mouse Motor Synapses.

We investigated the involvement of calcium-dependent enzymes, protein kinase C (PKC) and calcium-calmodulin-dependent protein kinase II (CaMKII), in the signaling pathway triggered by the activation of presynaptic alpha7-type nicotinic acetylcholine receptors by exogenous choline, leading to downregulation of the evoked acetylcholine (ACh) release in mouse motor synapses. Blockade of PKC with chelerythrine neither changed the evoked release of ACh by itself nor prevented the inhibitory effect of choline. The CaMKII blocker KN-62 did not affect synaptic activity but fully prevented the choline-induced downregulation of ACh release.

The mechanism of choline-mediated inhibition of acetylcholine release in mouse motor synapses.

The mechanism of action of tonically applied choline, the agonist of α7 nicotinic acetylcholine receptors (nAChRs), to the spontaneous and evoked release of a neurotransmitter in mouse motor synapses in diaphragm neuromuscular preparations using intracellular microelectrode recordings of miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) was studied. Exogenous choline was shown to exhibit a presynaptic inhibitory effect on the amplitude and quantal content of EPPs for the activity of neuromuscular junction evoked by single and rhythmic stimuli. This effect was inhibited either by antagonists of α7-nAChRs, such as methyllycaconitine and α-cobratoxin, or by blocking SK-type calcium-activated potassium (KCa) channels with apamin or blocking intraterminal ryanodine receptors with ryanodine. A hypothesis was put forward that choline in mouse motoneuron nerve terminals can activate presynaptic α7-nAChRs, followed by the release of the stored calcium through ryanodine receptors and activation of SK-type KCa channels, resulting in sustained decay of the quantal content of the evoked neurotransmitter release.

Involvement of basal and calcium-activated protein kinase C in neurotransmitter secretion in mouse motor synapses.

Blocker of presynaptic protein kinase C isoforms, GF109203X, reduced quantal content of single and rhythmic evoked end-plate potentials. The increase in quantal content of single potentials under the effect of 4- aminopyridine was neutralized by 75% under the effect of L-type Ca(2+)-channel blocker nitrendipine and completely returned to the control level after protein kinase C inhibition with chelerythrine. Neither nitrendipine, nor GF109203X affected the potentiating effect of tetraethylammonium on quantal content of end-plate potentials. Thus, we discovered basal activity of presynaptic protein kinase C under normal conditions that is aimed at the maintenance of quantal content of evoked release. It has been concluded that 4-aminipyridine, but not tetraethylammonium, triggers Ca(2+) entry into the terminal, which activates protein kinase C and enhanced the evoked acetylcholine release.

Facilitation of neurotransmitter release in mouse motor synapses in different modes of protein kinase C activation.

Protein kinase C blocker chelerythrine prevented the increase in quantal content of single and rhythmic evoked end-plate potentials after disinhibition of L-type Ca(2+)-channels with paxillin. Phorbol ester increased quantal content of single end-plate potentials and changed rhythmic activity of mouse motor synapses. The effects of phorbol ester were to a great extent neutralized by L-type Ca(2+)-channel blocker nitrendipine and were completely abolished by K(+)-channel blocker 4-aminopyridine. Thus, we discovered different facilitations of transmission after protein kinase C activation with calcium current through L-type channels and with phorbol ester.

Facilitation of acetylcholine secretion in mouse motor synapses caused by calcium release from depots upon activation of L-type calcium channels.

Pharmacological disinhibition of L-type Ca(2+) channels by two ways (with agonist S(-) BAY K 8644 and iberiotoxin, a Ca(2+)-activated BK-type K(+)-channel blocker) increases quantal content of evoked end-plate potentials, which was completely prevented by ryanodine (2 microM) blockade of ryanodine receptors. We conclude that increased quantal secretion of the transmitter induced by L-type Ca(2+) channel functioning requires activation of ryanodine receptors and calcium release from depots in motor terminals in mice.